Titanium oxides have been used as a white pigment having excellent hiding power and weather resistance for a long time. Recently, photocatalyst properties of titanium oxides are attracting attention. The photocatalyst generates radicals such as hydroxyl radicals and super oxide anion when UV is irradiated on the surface of the photocatalyst. The radicals adsorb or oxidatively decompose harmful substances (e.g. aldehydes), to decompose malodorous substances (substances regulated by the Offensive Odor Control Act), to prevent fouling or to sterilize. Recently, advances are made in applying photocatalysts as coatings to make use of the above functions. Many metal oxides are available as photocatalysts. Among them, anatase titanium oxides having high activity are used widely.
Meanwhile, low valence titanium oxides (TiOx; 0<x<2) such as Ti2O3 and the like are studied as inorganic black pigments being capable of replacing carbon black which is suspected to be a carcinogen (refer to Patent Document 1), as antistatic agent using their electrical conductivity and applied to materials such as paints, plastics, fibers and papers (refer to Non-patent Document 1), as oxygen absorbents (refer to Patent Document 2), as electrode materials of secondary cells (refer to Patent Document 3), and as materials that can expand the above-mentioned photocatalyst properties to the visible light range (refer to Patent Document 4).
Each of the above-mentioned properties exhibits a specific range of oxidation numbers corresponding to the usage of that property. For example, a composition with oxidation number, 0.2<x<1.95 is suitable as black pigment; a composition with oxidation number, x=1.75 exhibits a maximum electrical conductivity (refer to Non-patent Document 2); a composition with oxidation number, 1.5<x<1.9 is suitable as an oxygen absorbent; a composition with oxidation number, x=1, 1.67, 1.85<x<2 is suitable as an electrode material of a secondary cell; and a composition with oxidation number, 1.5<x<1.95 is suitable as a catalyst responsive to visible light.
The known methods of producing the low valence titanium oxides include a method of sintering metallic titanium and titanium dioxides under the present of nitrogen and a method of sintering titanium dioxides as the starting material in non-oxidizing or reducing atmosphere. In addition, a method of producing titanium monoxide using a pulsed plasma is disclosed (refer to Non-patent Document 3). Further, a method of producing low valence titanium oxides by reducing titanium dioxide at a temperature over 1000° C. in a hydrogen atmosphere is known (Non-patent Documents 4 to 6).
The method of producing low valence titanium oxides by reducing titanium dioxide can control the oxidation number to a certain extent by setting the reaction time or by other measures. However, the progress of the reduction process takes time to reach the interior of the particles, and further, processing at a high temperature generates sintered bodies and makes it difficult to obtain nano-particulates. Another problem in the reduction process is the use of explosive substances such as hydrogen at high temperatures.
Although an example exists of using titanium in electrodes for electrical discharging machining in view of its high melting point and high strength, oxide generation is difficult under the condition used in electrical discharging machining. In view of the background art, titanium-containing electrodes are almost never placed in water under conditions used in electrical discharging machining to produce titanium oxides. In fact, there is a report relating to methods for obtaining titanium monoxide particulates by the pulsed plasma method as shown in Non-patent Document 3, but no disclosures exist in relation to oxidation number control in titanium oxides generation.
Applications of titanium oxides as photocatalysts and electrical conductive materials are largely affected by the control of crystal morphology and the control of particle size distribution in addition to the control of oxidation numbers. The conventional methods of producing low valence titanium oxides mentioned above involve difficulties to inhibit any easy and stable production of an industrially acceptable level of low valence titanium oxides having a desired composition in addition to a uniform particle size distribution in the nano-order, and improvement was called for.